The present emphasis on the conversion of coal to substitute solid and liquid fuels has led to several alternative processes which are now being considered. The end use of the resultant converted coal will primarily determine the degree of conversion that must be accomplished and the quality of the desired product. The optimal use of the coal will depend on the specific application.
Among the many processes presently being considered is the solvent refining of coal (SRC) in which coal is treated at an elevated temperature in the presence of a hydrogen-donor solvent and hydrogen gas in order to remove the mineral matter, lower the sulfur content of the coal, and to convert it into a low melting solid which can be solubilized in simple organic solvents. This SRC can also be upgraded through catalytic hydrogenation to produce a liquid of higher quality. These two processes are of concern to the present invention.
Little is known at present as to the exact mechanisms by which the coal is transformed into soluble form, or of the detailed chemical structure of the soluble product or even the parent coal. It is known that many coals are easily solubilized and for others solubilization is more difficult. Some correlations have been made between the rank of the coal and ease of solubilization and product yield. A somewhat better correlation has been found with the petrography of the coal. Little is known about the relationships to product quality.
The initially dissolved coal (SRC) may have utility as a substitute clean fuel or boiler fuel; however, for substitute fuels of higher quality, specifications on viscosity, melting point, ash, hydrogen, and sulfur contents are much more stringent. Attempts to meet these specifications by operating the SRC process more severely have met with many difficulties such as low liquid yields, high hydrogen consumption, difficulty of separating unreacted residue, and excessive char formation, which often completely plugs process transfer lines and reactors.
Alternative methods of improving specifications through catalytic hydrogenation are also difficult. The problems which arise are threefold: (1) SRC components are susceptible to further condensation and may deposit as coke on catalysts used for their conversion, (2) they can also foul the catalysts by physical blockage as their size approaches the pore size of conventional catalysts and (3) they may contain metal contaminants, and their highly polar nature (particularly nitrogenous and sulfur compounds) can lead to selective chemisorption, and thus poison the catalysts.
The precise chemical nature of the SRC is still unknown; generally its composition is discussed in terms of solubility. Several classifications are commonly used. These include oils which are hexane or pentane soluble, asphaltenes which are benzene soluble, and pyridine soluble-benzene insoluble materials. Of these the asphaltenes and pyridine soluble-benzene insoluble materials are believed to be responsible for high viscosity, solvent incompatability, and processing difficulties. Little is known about the pyridine soluble-benzene insoluble materials. These have been referred to as "pre-asphaltenes" which implies that asphaltenes are derived from them; however, this has yet to be established.
More information is available on the nature of asphaltenes. It is common experience that coal liquids contain large quantities of materials known as asphaltenes. In fact, it has even been suggested that the formation of asphaltenes is a necessary step in the liquefaction of coal.
The term asphaltene is a rather nebulous and all-inclusive classification of organic materials for which a detailed chemical and physical identification is quite difficult, and has not yet been accomplished.
This classification generally refers to high molecular weight compounds, boiling above 650.degree. F., which are soluble in benzene and insoluble in a light paraffinic hydrocarbon (e.g., pentane). Usually no distinction is made regarding polarity, as the term has been used customarily in the characterization of heavy petroleum fractions (resids, etc.) where the amount of highly polar materials is small. However, in coal liquids this may not necessarily be the case due to the high degree of functionality of coal itself. Thus, coal liquids of low molecular weight may still be "asphaltenes." There is considerable variation in the molecular weight of solubilized coals which arises from differences in the parent coals, or different solvent or solvent-reactant systems at the same temperature of reaction. This could well be related to colloidal properties of coal liquids. It is well documented that asphaltenes found in heavy petroleum fractions are colloidal in nature.
Some comments on the chemical nature of coal asphaltenes have recently been made. Asphaltenes from Synthoil Process liquids were separated into a basic fraction (containing oxygen only as ether or ring oxygen and basic nitrogen as in pyridine) and an acidic fraction (containing phenolic OH and nitrogen as in pyrrole). The two fractions were found to have very different properties. The basic fraction could be hydrotreated only with difficulty, while the acid fraction underwent facile hydrotreating. This is consistent with reported data on the influence of nitrogen heterocycles on conventional hydroprocessing.
Based on these results an acid-base pair structure for asphaltenes was proposed and this structure was extrapolated to that of coal itself. This structure is quite different from the more amphoteric nature of coal which has been proposed previously.
Mechanisms have been proposed for the noncatalyzed formation of asphaltenes from coal. In this work it was concluded that asphaltenes were a necessary product of coal liquefaction and that oils were derived from asphaltenes. The more polar pyridine soluble materials were not investigated and were assumed to be equivalent to unreacted coal. The maximum yield of asphaltenes was found, however, to be a function of the conditions of coal conversion; hydrogen donor solvents greatly reduced the propensity for formation of asphaltenes at low conversion. In addition, it was not determined whether the asphaltene fractions resultling from different conditions were of the same chemical and/or physical nature. Thus, asphaltenes may be inherent constituents of coal products or they could well be the result of either thermal or catalytic transformations of more polar materials.
In considering what may be involved in the formation of asphaltenes during coal solubilization or conversion, it may be instructive to consider what is known of coal structure. Coal is a rather complicated network of polymeric organic species, the bulk of which is porous in the natural form; the pore system varies from coal to coal. Depending upon the specific nature of the porous structure of each coal, its chemical constituents, and the reaction conditions, the rate of diffusion and mass transport of organic molecules through the pores could have a strong effect on the rates of dissolution, hydrogen transfer, and hydrogenation and hydrocracking reactions, and thus on the ultimate yield of soluble product.
As the rank of coal becomes higher, an increasing number of colloidal size aggregates (20-50 A) can be observed by X-ray scattering and diffraction.
If, in the early stages of the dissolution of coal these colloidal aggregates dissociate to some degree and go into solution, the molecular weight of the lowest unit appears to be consistent with the lowest molecular weights observed in solubilized coals (.about.500 MW). This comparison may be coincidental, however. Unfortunately, in order to dissolve coal it is generally found that temperatures in excess of 300.degree. C. are necessary. It is also known that coal begins to pyrolize and evolve volatile matter at temperatures as low as 250.degree. C. (depending on rank), and by 350.degree. C. considerable material has evolved. This strongly suggests that extensive internal rearrangement of the coal occurs during the dissolution process. Rearrangement can include hydrogen migration to produce highly condensed aromatic rings as well as further association of small colloidal aggregates or condensation of reactive species. Major physical changes in the pore system of the solid coal have also been reported.
This rearrangement could possibly be responsible for some of the very high molecular weights (.about.3000 MW) observed with some solvents. No detailed relationships of solvent type and/or reaction condition to the molecular weight distribution of solubilized coal has yet been established. Similarly, the possibility of reversible molecular weight changes, due to recondensation causing increased molecular weights at various temperatures, has not been investigated thoroughly.
An alternative route to high molecular weight is through the catalytic influence of inorganic coal minerals which are present in the processing of coal. It is known that some coals are more reactive than others, producing higher yields of liquid products at shorter residence times. It is believed that this is due to the fact that the initial coal products are reactive and condense to char unless proper reaction conditions are established. This further condensation could well be a catalytic phenomenon induced by intrinsic coal minerals.
Another more subtle consequence of certain inorganic constituents is their influence on the physical properties of pyrolytic coal chars, and thus on the diffusional properties imposed on reactive intermediates. The volume of char has been observed to vary by a factor of four or more, with little change in weight, by varying the type of inorganic contaminants in a given bituminous coking coal. The pore system of the resultant chars must be vastly different and changes of this type magnitude in the physical structure of the coal or char could greatly influence mass transport of intermediates produced within the pore system. Mass transfer limitation during the pyrolysis and hydrogasification of some coals at high temperatures has recently been established. This study showed that for some coals, reactive primary products are formed which can recombine to produce char if the conditions are not properly adjusted. The criticality was found to be the rate of diffusion of the reactive species out of the coal relative to its rate of conversion to char.
At lower temperatures, the rates of reaction are, of course, slower and thus less susceptible to mass transport limitations. However, the imposition of a liquid phase, commonly used in liquefaction processes, may greatly enhance diffusional restrictions. Recent model studies conducted in aqueous systems, have shown that restriction of diffusion through porous structures with pore radii ranging from 45 A to 300 A for even relatively small solute molecules is very significant.
At the present stage of the art, the accumulated information is largely empirical, with little basis for sound extrapolation to predict detailed nature of solvent and processing conditions for optimum yield and quality of solvent refined coal. It is recognized that the poorly understood asphaltenes are probable sources of many of the problems encountered, e.g. formation of char at processing conditions conducive to efficient separation of mineral matter (ash) and sulfur from desired product at high yield.
In the process of converting coal to a low sulfur, low melting solid by use of recycled product fractions as solvent, several reaction steps occur. Generally coal is admixed with a suitable solvent recycle stream and hydrogen and the slurry is passed through a preheater to raise the reactants to a desired reaction temperature. For bituminous coal, the coal is substantially dissolved by the time it exits the preheater. Sub-bituminous coals can be dissolved but care must be exercised not to raise the temperature too high and thus promote charring.
The products exiting from the preheater are then transferred to a larger backmixed reactor where further conversion takes place to lower the heteroatom content of the dissolved coal to specification sulfur content and melting point. The geometry of this reactor is such that the linear flow rate through it is not sufficient to discharge a substantial quantity of particulate matter of a desired size. Thus the reactor volume becomes filled (at steady state) up to about 40 vol % by solids which are produced from the coal. These solids have been shown to be catalytic for the removal of heteroatoms and the introduction of hydrogen into the coal products and solvent. The products exiting the reactor are initially separated by flash distillation, which depressurizes the stream and removes gases and light organic liquids. The products are further separated (filtration, centrifugation, solvent precipitation, etc.) and the filtrate is distilled to recover solvent range material (for recycle) and the final product SRC.